In recent years, cars have been equipped with an idling stop function which stops an engine when the car comes to a stop, or an electric power steering wheel which takes the load off the engine. These two items contribute environmental protection and fuel saving. A hybrid system or an electric turbo system, which positively complements the drive of engine, will be used widely in the market. On top of that, car manufacturers have proposed various ideas about a car brake such as an electrical hydraulic brake that will replace a conventional mechanical hydraulic brake.
As discussed above, the car tends to need electric power increasingly from now on; however, a battery, having conventionally supplied power to the car, cannot supply an instantaneous large amount of power only by itself, so that it sometimes fails to supply sufficient power. If the battery becomes abnormal, the driving system possibly fails to work normally.
To overcome the foregoing problems, an electric storage device is proposed as an auxiliary power supply for supplying enough power when the battery falls into abnormal operation. The electric storage device is disclosed in, e.g. patent document 1, which refers to the electric storage device, in particular, a backup power-supply unit for supplying power to an electronic controller of a system when the battery falls into abnormal operation.
FIG. 14 shows a block diagram of a conventional electric storage device. In FIG. 14, an electrically double-layered capacitor having large capacitance is used as a capacitor for storing electric power. Multiple capacitors are coupled together to form capacitor unit 101 working as an electricity storing section. Capacitor unit 101 has charging circuit 103 and discharging circuit 105 coupled thereto for controlling the charging and discharging of capacitor unit 101, while these circuits are controlled by microprocessor 107. Voltage sensor 109 is connected to microprocessor 107 for sensing abnormal operation of a battery, while it is coupled to FET switch 111 which supplies power to capacitor unit 101 when sensor 109 senses abnormality.
Electric storage device 113 structured above and working as a backup power-supply unit is coupled between battery 115 and electronic controller 117 working as a load. Storage device 113 is controlled its start and halt by ignition switch 119.
Assume that electronic controller 117 is employed in an electric braking system of a vehicle, then controller 117 must be kept driving to allow applying a brake when battery 115 falls in abnormal operation. In such a case, when voltage sensor 109 senses an abnormality of battery 115, FET switch 111 is turned on so that capacitor unit 101 can supply power to controller 117, thereby overcoming the abnormality of battery 115.
Capacitor unit 101 basically works as an auxiliary power supply with the structure and operation discussed above. However, since the electrically double-layered capacitor forming capacitor unit 101 is degraded time-dependently, capacitor unit 101 needs to be monitored its degradation in order to drive controller 117 at any time, and a degradation should be reported to an operator. The electric storage device thus needs the foregoing functions in order to maintain highly reliable operation. The conventional electric storage device thus monitors the changes in its internal resistance value “R” and capacitance “C” which vary in response to the degradation of capacitor unit 101.
Since the values of internal resistance “R” and capacitance “C” are found when capacitor unit 101 is charged, a method of charging the capacitor unit 101 is firstly described hereinafter. FIG. 15 shows variation with time in the voltage of capacitor unit 101 during the charge to the conventional electric storage device. The horizontal axis represents time “t” while the vertical axis represents voltage “V” of capacitor unit 101. In FIG. 15, charging circuit 103 supplies a given current “I” at time “t0” from battery 115 to capacitor unit 101 in order to charge capacitor unit 101. At this instant, voltage “V” rises proportionately to internal resistor R of capacitor unit 101, and then voltage “V” rises linearly due to a charge with a constant current as shown in FIG. 15.
Charging circuit 103 interrupts the charge temporarily (e.g. at time “t1”) in the course of the charging, so that voltage “V” lowers proportionately to internal resistance “R” as shown in FIG. 15. However, since capacitor unit 101 has stored electric charges, voltage “V” will not lower more than a value caused by internal resistor “R”before it settles down at a certain value. Then the charge starts again at time “t2”, and voltage “V” rises proportionately to internal resistor “R” as it has risen at time “t0”. The voltage “V” linearly rises in the course of the charge before capacitor unit 101 is fully charged at time “t3”. The charge then halts and voltage “V” stays at a certain value.
Capacitor unit 101 is thus charged and the values of its internal resistor “R” and capacitance “C” are found in the course of the charge. First, internal resistor “R” can be found by measuring the rises of voltage “V” at time “t0” and “t2” or the fall of voltage “V” at time “t1”. A voltage sensing section built in charging circuit 103 can find these rises or a fall in the voltage.
To be more specific, the range of rise or fall (hereinafter referred to as a voltage variable range “ΔV”) of voltage “V” changes proportionately to internal resistance “R”, so that the voltage sensing section finds the voltage variable range “ΔV” at any one of time “t0”, “t1”, and “t2”. Since current “I” to be used for charging capacitor unit 101 has a known and predetermined value, internal resistance “R” can be found by the equation: ΔV=R×I. The voltage variable range “AV” can be found at any one of time “t0”, “t1”, and “t2”, however, since time “to” comes right after the start, a greater measurement error can be expected, so that the variable range “ΔV” found at time “t1” or “t2”, at which the charge is interrupted temporarily, is preferably used.
Next, capacitance “C” is found from an inclination “V/t” during the time span of t0-t1 or t2-t3 in the graph shown in FIG. 15. To be more specific, electric charge amount “Q” of capacitor unit 101 can be found by the equation: Q=C×V, while Q=I×t is established, so that C=I×(t/V) is satisfied. Accordingly, capacitance “C” can be found by multiplying an inverse number of the inclination V/t of the graph by current “I”. In the foregoing discussion, the way of finding “R” and “C” during the charge of capacitor unit 101 is described; however, they can be found in a similar way during the discharge, with a constant current, from capacitor unit 101.
The “R” and “C” thus found are compared with a degradation limit found in advance, thereby determining how much the capacitor unit 101 is degraded. A reliable electric storage device has been thus obtained.
The foregoing conventional electric storage device can indeed determine how much its capacitor unit 101 is degraded, and thus maintain the reliability at a high level, but the “R” and “C” can be actually found during the charge or discharge only with a constant current. If the load coupled to the electric storage device is, e.g. a motor of a hybrid system, capacitor unit 101 undergoes charges/discharges with a large current repeated frequently in a short time. The conventional method is thus not suitable for finding internal resistance “R” among others.                Patent Document 1: Japanese Unexamined Patent Publication No. 2005-28908        